This invention relates to a measuring system of transmission characteristics, and more specifically to a measuring system of transmission characteristics for measuring group delay characteristics (and chromatic dispersion characteristics) of an optical fiber line and optical element.
Chromatic dispersion characteristics in an optical fiber transmission line seriously affect transmission characteristics. The chromatic dispersion means a group delay amount per wavelength and thus the chromatic dispersion characteristics can be derived by differentiating group delay characteristics with wavelengths. Accordingly, in general, a system for measuring chromatic dispersion characteristics actually measures the group delay characteristics of each wavelength and then calculates the chromatic dispersion from the obtained result.
The group delay itself can be measured from a difference of propagation time per wavelength. As to a long optical fiber, there are two kinds of measuring methods; one uses an arrival time difference of a single pulse and the other is a phase-shifting method for detecting a phase variation of a periodic signal such as a sine wave. In the phase-shifting method, a reference phase for comparing phases is required, and reference and measuring signals are used. When input and output ends of an optical fiber are disposed at the same place, the reference signal is entered directly to a phase comparator and the measuring signal alone is transmitted on the to-be-measured optical fiber and then input the phase comparator. By sweeping a wavelength of the measuring signal in that state, group delay characteristics (propagation time differences relative to wavelengths) can be measured. On the other hand, when the input and output ends are disposed at different places such as an already installed optical fiber (including an optical amplifying system), the reference signal should be transmitted toward a remote end side.
The system for measuring the chromatic dispersion by transmitting the reference and measuring signals through the to-be-measured fiber is disclosed in Japanese Patent publication Gazette No. Heisei 7-9386 and a specification of U.S. Pat. No. 4,984,884. Naturally, the optical wavelengths of the reference and measuring signals should be different and therefore a wavelength multiplexer for wavelength-multiplexing the reference and measuring signal lights is disposed on the input side of the to-be-measured fiber and a wavelength demultiplexer for wavelength-demultiplexing the reference and measuring signal lights is disposed on the output side of the to-be-measured fiber. The reference and measuring signal lights are modulated by an output from a common oscillator. The reference and measuring signal lights separated by the wavelength demultiplexer are converted into electric signals respectively and then a group delay amount of the measuring signal can be measured by comparing phases of those two electric signals. Also, wavelength characteristics of the group delay, namely chromatic dispersion characteristics can be obtained by sweeping the wavelength of the measuring signal light.
However, since the optical amplifying transmission system has a quite narrow transmission wavelength bandwidth, a wavelength demultiplexer with excellent spectral characteristics is required in order to measure its chromatic dispersion characteristics, and as a result its apparatus becomes expensive. When a spectral wavelength of the wavelength demultiplexer is fixed, wavelength bands allowably to be measured are limited, on the other hand, when the spectral wavelength is variable, the reference and measuring signals are required to follow the varied spectral wavelength causing further increase of the manufacturing cost.
An object of the present invention is to provide a measuring system of transmission characteristics for measuring group delay or chromatic dispersion characteristics more simply and accurately than ever.
Another object of the present invention is to provide a measuring system of transmission characteristics not requiring any wavelength demultiplexer.
According to the invention, a measuring system of transmission characteristics comprises a reference signal light generator for generating a reference signal light of a reference wavelength xcexr optically intensity-modulated at a reference frequency fr, a measuring signal light generator for generating a measuring signal light of a measuring wavelength xcexs optically intensity-modulated at a measuring frequency fs synchronizing the reference frequency fr, a multiplexer for multiplexing the reference signal light and measuring signal light generated respectively by the reference signal light generator and measuring signal light generator and outputting them onto a to-be-measured optical line, a photodetector for converting the lights propagated on the to-be-measured optical line into electric signals, a reference frequency extractor for extracting the component of the reference frequency fr from the output of the photodetector, and a phase detector for detecting a phase of the frequency component corresponding to the measuring frequency fs contained in the output of the photodetector according to the output of the reference frequency extractor.
With this configuration, the transmission characteristics at the measuring wavelength can be measured without demultiplexing the wavelengths of the reference signal light (reference wavelength) and measuring signal light (measuring wavelength). The reference and measuring wavelengths can be closely disposed because it is not necessary to demultiplex the wavelengths.
The phase detector preferably comprises a phase-locked oscillator for generating a signal synchronizing the output of the reference frequency extractor and having a frequency corresponding to the measuring frequency fs, and a detector for detecting, in accordance with the output of the phase-locked oscillator, a phase of a frequency component corresponding to the measuring frequency fs included in the output of the photodetector. The detector preferably comprises a synchronous detector.
When at least one of a signal showing the measuring wavelength xcexs and a trigger signal showing timing of change for the measuring wavelength xcexs is transmitted after being superimposed on either of the reference frequency fr and measuring frequency fs, the variation of the measuring wavelength can be detected at a receiving side of the measuring signal light.
Also, the measuring system of transmission characteristics according to the invention comprises an oscillator for oscillating at the measuring frequency fs, a first signal light generator for generating a first signal light of a first wavelength xcexa optically intensity-modulated at the measuring frequency fs and outputting it onto a first optical line, a first photodetector for receiving the first signal light propagated the first optical line and outputting a signal at a frequency corresponding to the measuring frequency, a second signal light generator for generating a second signal light of a second wavelength xcexb optically intensity-modulated with the frequency signal from the photodetector and outputting it onto a second optical line, a second photodetector for receiving the second signal light propagated the second optical line, and a phase detector for detecting a phase of the measuring frequency component contained in the output of the second photodetector according to the output of the reference oscillator.
In this configuration, the respective transmission characteristics of the first and second optical lines can be individually measured by controlling the first and second wavelengths. Moreover, the terminal equipment on one side alone is sufficient for the measurement.